One well known type of cylindrical baler comprises at least one set of longitudinally extending side-by-side belts supported on a plurality of transverse rollers. The belts and rollers define an expandable chamber for forming cylindrical, spirally round bales of crops such as hay and forage. To form a bale, the crops are picked up from the ground as the baler moves across the field. When the bale reaches a predetermined diameter, it is bound by being wrapped in twine, netting or the like in the bale forming chamber. The bale is then discharged onto the ground through a rear tailgate. A hydraulic circuit is selectively operated to maintain tension in the compacting belt arrangement during bale formation and to open and close the tailgate following bale formation.
One example of a cylindrical baler of this type is disclosed in U.S. Pat. No. 4,545,298 issued Oct. 8, 1985 to Viaud. In this arrangement, a directional control valve, an adjustable discharge valve, a non-return valve, and a pressure limiting valve are all required to operate between a pair of cylinders to control variable tension on the bale and to open and close the tailgate. Another example of this basic arrangement is shown in U.S. Pat. No. 5,025,718 issued Jun. 25, 1991 to Viaud et al. In this disclosure, a directional control valve, a bypass solenoid valve, an adjustable high pressure relief valve, and a gate valve operate in concert to form variable density bales and provide actuation of the tailgate. The solenoid valve as used in the latter arrangement is employed primarily to regulate hydraulic fluid direction rather than control hydraulic fluid pressure.
Realizing the limitations of the prior art, it is desirable to improve upon the aforementioned arrangements by eliminating the directional control valve and by employing a single proportional valve which maintains a predetermined pressure of the tensioning cylinder or between the tensioning cylinder and the tailgate cylinder, and controls, inter alia, the density of the formed bales.
Accordingly, it is a general aim of the invention to supply a hydraulic circuit employing a proportional valve and a series of check valves to control oil flow and pressure to and from the hydraulic cylinders which apply tension to the bale forming arrangement and open/close the tailgate before and after bale ejection. It is further desirable to provide a hydraulic circuit in which one power source is utilized to maintain tension on the compacting belts, open and close the tailgate during bale ejection, and positively return the belt take-up device to the start position of the bale forming cycle. It is also within the purview of the invention to provide a solenoid type proportional valve which allows a bale density setting to be made from a control unit remote from the baler.